In recent years, high pressure acid leach using sulfuric acid has been attracting attention as a hydrometallurgical method for nickel oxide ores. Unlike pyrometallurgy, which is a conventional common refining method for nickel oxide ores, the high pressure acid leach does not include a pyrometallurgical step using reduction and drying plants, but includes a consistent hydrometallurgical step, and thus is advantageous in terms of energy and cost. In addition, this method has another advantage that a sulfide containing nickel and cobalt (hereinafter, sometimes referred to as “a nickel-cobalt mixed sulfide” or “a Ni—Co mixed sulfide”) whose nickel grade is improved up to approximately 50% by mass can be obtained.
Examples of a plant to perform a nickel refining treatment by high pressure acid leach include: (a) a leaching and solid-liquid separation plant configured such that sulfuric acid is added to a slurry of a nickel oxide ore to perform a leaching treatment under high temperature and high pressure, and subsequently, multistage washing is applied to a leach slurry, whereby a residue is separated therefrom to obtain a leachate containing an impurity element together with nickel and cobalt; (b) a neutralization plant configured such that the pH of the obtained leachate is adjusted to separate a neutralized precipitate containing the impurity element therefrom, whereby a post-neutralization solution containing zinc together with nickel and cobalt is obtained; (c) a dezincification plant configured such that a sulfurizing agent is added to the post-neutralization solution thereby to form a zinc sulfide, and the zinc sulfide is separated therefrom to obtain a leachate containing nickel and cobalt; and (d) a nickel recovery plant configured such that a sulfurizing agent is added to the leachate thereby to form a mixed sulfide containing nickel and cobalt, and the mixed sulfide is separated therefrom.
Here, in the foregoing dezincification plant (c), the post-neutralization solution discharged from the neutralization plant is introduced into a sulfurization reaction tank, and a sulfurizing agent, such as hydrogen sulfide gas or sodium hydrosulfide, is added thereto to sulfurize zinc, copper, and the like which are contained in the post-neutralization solution. After that, solid-liquid separation is performed using a filter press or the like, whereby a zinc sulfide and a leachate containing nickel and cobalt are obtained. (For example, refer to Patent Literatures 1 and 2.). The nickel-cobalt mixed sulfide is further used as a raw material for purification to obtain electrolytic nickel and electrolytic cobalt, and therefore, the treatment in the dezincification plant requires the concentration of zinc (Zn) contained in the post-neutralization solution to be reduced to not more than 1 mg/L.
Therefore, to the nickel-and-cobalt-containing leachate obtained by the solid-liquid separation of the zinc sulfide with a filter press or the like in the dezincification plant, a further filtration treatment is applied so that a minute zinc sulfide precipitate which cannot be separated by the solid-liquid separation is removed. As a filter for the filtration treatment, for example, a polishing filter is used.
Commonly, a start-up operation of a dezincification plant including a filter after the completion of a plant periodic inspection or the like is performed in such a manner that the flow rate of a liquid transfer pump is set to be low in the beginning, and then, the flow rate is gradually and continuously increased to transfer a slurry in a long time, and, at the timing when a certain amount of a zinc precipitate is coated on the surface of a filter cloth provided to the filter, the flow rate is made to reach a flow rate in a normal operation (a target flow rate). Then, after a shift to the normal operation, what is called cake filtration is performed on the basis of the coating layer (cake layer) of the zinc precipitate formed on the surface of the filter cloth.
However, as mentioned above, the coating of the surface of the filter cloth with the precipitate takes a long time. Specifically, for example, in a factory (a plant) capable of producing a Ni—Co mixed sulfide on a scale of approximately 10,000 tons per year (in terms of the volume of nickel), the coating with the precipitate sometimes takes a long time, namely, approximately one day. This causes a considerable decrease in the operating rate and a decrease in the volume of production of a Ni—Co mixed sulfide.
That is, during the coating of the surface of the filter cloth with the precipitate, the flow rate of the slurry transferred to the filter of the dezincification plant is lower than the flow rate in a normal operation, and, accordingly, the throughput in the dezincification plant decreases. Hence, in response to the decrease in throughput in the dezincification plant, the operating rate of the whole process including the neutralization plant used in the upstream step needs to be decreased, and as a result, the volume of production is considerably decreased. Specifically, for example, in the case of the plant capable of producing a Ni—Co mixed sulfide on a scale of approximately 10,000 tons per year (in terms of the volume of nickel), at the time of start-up of the dezincification plant, the operating rate of the whole process needs to be decreased to approximately 80%.
At this time, if a measure of decreasing the operating rate of the whole process is not taken, then, a slurry which should be transferred to the dezincification plant (a slurry which cannot be treated in time) overflows from a buffer tank provided between the dezincification plant and the neutralization plant used in the upstream step. Furthermore, in a buffer tank between the dezincification plant and the nickel recovery plant used for the downstream step, a slurry to be accommodated runs short, and the plant operating rate decreases, accordingly.
To prevent such decrease in production volume, for example, there can be taken a measure of installing buffer tanks larger than ever at the points ahead and behind of a filter, or a measure of installing more filters. However, these measures have problems that a high initial investment is required, thereby reducing economic efficiency, and furthermore, there is a limit to installation space.
Alternatively, there can be considered a measure of extremely increasing a supply flow rate after the coating (called “ramp-up”), but, this measure causes a problem that an excessive load is applied to a purification filter. Alternatively, there can be mentioned a measure of rapidly supplying a slurry from the time of plant start-up at the maximum pumping capacity, but, a formed cake layer is not good because the layer is almost in a state of clogging due to the zinc sulfide which coats the layer, and accordingly, filtering accuracy decreases. Furthermore, in these cases, the life of the filter cloth is considerably shortened, whereby the frequency of replacement of a filter cloth, the frequency of maintenance thereof, and the like are increased, and accordingly, treatment efficiency is further lowered.
Thus, from a viewpoint of securing a stable production volume, the feasibility of any of the foregoing measures is low.